1. Field of the Invention
The present invention relates to a printing method and a printing apparatus for carrying out the same and, more specifically, to a thermal printing method and a thermal printing apparatus for carrying out the same.
2. Description of the Related Art
The recent progressive development of color image transmission and recording by television cameras, television systems and computer graphic systems have sharply increased need to print color images in color pictures on recording media, and color printers of various printing systems have been developed and applied to various fields.
A color printer of a thermal dye-transfer printing system, which is one of the previously proposed color printers of various printing systems, presses an ink sheet formed by coating a sheet with a layer of ink prepared by dispersing a dye in a high density in a binder resin and a recording medium, such as a recording sheet formed by coating a sheet with a dye-accepting resin that accepts the dye, closely against each other, applies heat to the ink sheet according to image information with a thermal print head placed on the ink sheet or with a laser beam emitted by a laser light source so that a quantity of the dye proportional to the quantity of heat applied to the ink layer is transferred from the ink sheet to the recording medium. A thermal sublimable dye printing method employing a sublimable dye or a heat-diffusible dye can be carried out by a printing apparatus having a comparatively small size and requiring simple maintenance service. A printer of the so-called thermal printing system, which prints a full-color picture having continuous gradation corresponding to the amount of heat energy by repeating the foregoing printing cycle for image signals representing images of the three subtractive primaries, namely, yellow, magenta and cyan, has a capability of immediately printing a color picture in a high picture quality comparable to that of silver salt photographs.
FIG. 17 is a schematic front view of an essential portion of a thermal printer of such a thermal printing system. A thermal print head 91 is disposed opposite to a platen roller 93. An ink sheet 92 formed by coating a base film 92b with an ink layer 92a, and a recording sheet 100 formed by coating a paper sheet 100b with a dyeing resin layer 100a are held between the thermal print head 91 and the platen roller 93 and pressed against the platen roller 93 by the thermal print head 91. The platen roller 93 is rotated to feed the ink sheet 92 and the recording sheet 100. Portions of the ink layer 92a are heated locally and selectively by the thermal print head 91 to transfer the ink, i.e., a printing material, contained in the ink layer 92a to the dye-accepting resin layer 100a of the recording sheet in dots for printing. Generally, such a thermal printer is of a line printing system provided with an elongate thermal print head disposed with its length extending perpendicularly to the direction of feed of the recording sheet.
The ink sheet employed in the foregoing conventional thermal sublimable dye printing method is a throw-away ink sheet formed by coating a base sheet, such as a polyester film, with a dye layer of a mixture of a dye and a binder resin having a dye-to-resin weight ratio of about 1:1, having a thickness on the order of 1 .mu.m. Therefore, the use of this ink sheet entails problems in resources conservation and environmental protection. To improve the utilization of such an ink sheet by repeatedly using the same, there have been proposed, for example, a dye layer regenerating method which replenishes the used dye layer with the dye, a multidye layer forming method which forms a multidye layer consisting of a plurality of laminated dye layers, and a relative speed control method which controls the ink sheet feed speed relative to the recording sheet feed speed to increase the amount of prints which can be printed with a unit length of the ink sheet.
All the conventional thermal printing methods press the dye layer of the ink sheet against the dye-accepting layer of the recording sheet and heat the dye layer of the ink sheet. For example, when printing a color picture by the conventional thermal printing method, an yellow ink sheet is superposed on a recording sheet with the yellow dye layer thereof in contact with the dye-accepting layer of the recording sheet and the yellow ink sheet is heated to form a yellow picture on the recording sheet, a magenta ink sheet is superposed on the recording sheet with the magenta dye layer thereof in contact with the dye-accepting layer of the recording sheet and the magenta ink sheet is heated to superpose a magenta picture and the yellow picture on the recording sheet, a cyan ink sheet is superposed on the recording sheet with the cyan dye layer thereof in contact with the dye-accepting layer of the recording sheet and the cyan ink layer is heated to superpose a cyan picture, the yellow picture and the magenta picture on the recording sheet, and, when need be, a black ink sheet is superposed on the recording sheet with the black ink layer thereof in contact with the dye-accepting layer of the recording sheet and the black ink sheet is heated to superpose a black picture, the yellow picture, the magenta picture and the cyan picture on the recording sheet to form a color picture.
Thus, the conventional thermal printing method prints pictures respectively having different colors successively by pressing a dye layer having a color different from those of the previously printed pictures against the previously printed pictures when printing a color picture. Therefore, it occurs sometimes that the dyes previously printed on the recording sheet are transferred from the recording sheet to the dye layer of an ink sheet for printing the next picture to deteriorate the picture quality and to contaminate the dye layer of the ink sheet for printing the next picture. When the ink sheet is used repeatedly, the contamination of the dye layer thereof is a significant problem.